Control of currents in glass drawing kiln

ABSTRACT

In a glass drawing kiln containing a mass of molten glass from which a glass ribbon is drawn, the uniformity of the temperature of the glass feeding the ribbon is improved by locally heating the glass in one region of the kiln adjacent, but spaced from, a kiln wall to produce a rising current of heated glass which forms a barrier across which glass adjacent such wall is prevented from flowing and which supplies glass having an increased temperature to the ribbon.

E. BRICHARD 3,834,915 CONTROL OF CURRENTS IN GLASS DRAWING KILN 1.2 Sheets-Sheet 1 Filed Feb. 24; 1972 I I l Tflij Fig.2.

Se t. 10, 1974 E. BRICHARD CONTROL OF CURRENTS IN GLASS DRAWING KILN LZ'Sheets-Sheet 2 Filed Feb. 24, 1972 q 10, 1974 "E. BRICHARD I 3,834,975

CONTROL OF CURRENTS IN GLASS DRAWING KILN Filed Feb. 24, 1972 12 Sheets-Sheet 3 Sept. 10, 1974 E. BRICHARD CONTROL OF CURRENTS IN GLASS DRAWING KILN Filed Feb. 24, 1972 12 Sheets-Sheet 4 p 10, 1974 E. BRICHARD 3,834,975

CONTROL OF CURRENTS IN GLASS DRAWING KILN Filed Feb. 24, 1972 LLIJI'I 1 III 111 r 12 Sheets-Sheet 5 Filed Feb. 24, 1972 l 1974 E. BRICHARD 3,834,975

CONTROL OF CURRENTS IN GLASS DRAWING KILN 12 Sheets-Sheet 6 o q 10, 1974 E. BRICHARD 3,834,975

CONTROL OF CURRENTS IN GLASS DRAWING KILN Filed Feb. 24, 1972 l2 Sheets-Sheet 7 p 0, 1974 E. BRICHARD 3,834,975

CONTROL OF CURRENTS IN GLASS DRAWING KILN 7 Filed Feb. 24, 1972 12 Sheets-Shet 6 168\ I7OAEI Ham 3 I67 Sept. 10, 1974 E. BRICHARD CONTROL OF CURRENTS IN GLASS DRAWING KILN 12 Sheets-Sheet 9 Filed Feb. 24, 1972 Sept. 10, 1974 E. BRICHARD CONTROL OF CUHRENTS IN GLASS DRAWING KILN l2 Sheets-Sheet 10 Filed Feb. 24, 1972 p '1974 E. BRICHARD 3,834,975

CONTROL OF CURRENTS IN GLASS DRAWING KILN Filed Fb. 24, 1972 12 Sheets-Sheet 11 Sept. 10, 1974 E.,BR\ICHARD CONTROL ,OF CURRENTS IN GLASS DRAWING KILN 12 Sheets-Sheet 12 Filed Feb. 24, 1972 Fig 25 United States Patent 3,834,975 CONTROL OF CURRENTS IN GLASS DRAWING KILN Edgar Brichard, Jumet, Belgium, assignor to Glaverbel S.A., Watermael-Boitsford, Belgium Filed Feb. 24, 1972, Ser. No. 228,963 Claims priority, application Luxembourg, Feb. 24, 1971, 62,648; Great Britain, Jan. 24, 1972, 3,278/ 72 Int. Cl. C03b 15/04 U.S. Cl. 161-1 52 Claims ABSTRACT OF THE DISCLOSURE In a glass drawing kiln containing a mass of molten glass from which a glass ribbon is drawn, the uniformity of the temperature of the glass feeding the ribbon is improved by locally heating the glass in one region of the kiln adjacent, but spaced from, a kiln wall to produce a rising current of heated glass which forms a barrier across which glass adjacent such wall is prevented from flowing and which supplies glass having an increased temperature to the ribbon.

BACKGROUND OF THE INVENTION This invention relates to improvements in the drawing of sheet glass, such drawing being basically carried out by continuously feeding molten glass into a kiln to establish a continuous forward flow of glass to a drawing zone at which molten glass is continuously upwardly drawn from the surface of the molten glass in the kiln in the form of a ribbon. The invention also relates to apparatus for effecting such improvements and to the novel sheet glass resulting therefrom.

In the performance of a glass drawing process as referred to above, the thermal and flow conditions in the kiln are of critical importance to the quality of the drawn glass. It is in all cases necessary for these conditions to be such that a substantially stable meniscus is established at the surface of the glass at the drawing zone; but the formation and maintenance of such a meniscus by no means ensures that the drawn glass will be of the best quality.

As a drawing operation progresses, glass is drawn inwardly into the meniscus from surface regions of the molten glass surrounding the meniscus and the glass temperature variations which invariably exist between surface regions of the glass at difierent distances from boundary walls of the kiln, combined with the rather complex flow pattern in the kiln, tend to prevent the formation of a ribbon which is truly flat and of substantially uniform thickness over its width, and also to lead to optical defects due to the mixing in the ribbon of currents composed of glass of different viscosities. These tendencies become more marked as the drawing speed increases.

The problems referred to arise in all drawing processes in which the glass in drawn from the surface of the molten glass in the kiln, as distinct from processes in which the molten glass is extruded into the ribbon from beneath the surface of the molten glass in the klin as in the classic Fourcault process. In such extrusion processes the flow pattern of the glass is quite different and the problems above referred to do not arise.

Broadly speaking, the drawing processes with which the present invention is concerned can be divided into two categories according to the depth of the kiln at the drawing zone. On the one hand, use can be made of a shallow kiln, or pot, from which glass is drawn from the full depth of molten glass at the drawing zone. This category of process includes the classic Colburn process in which the glass ribbon drawn upwardly from the kiln is bent over a bending roller and is conveyed through a horizontal annealing lehr. On the other hand, use can be made of a deep Ikiln, or tank, in which the forward current of glass flowing to the drawing zone flows over a return current of colder glass coming from the terminal, or downstream, end region of the kiln. This category of process includes the classic Pittsburgh process in which the glass ribbon is drawn upwardly through a vertical drawing tower.

Numerous modifications of these processes are possible within the broad categories referred to. For example the glass ribbon can in any given type of process be drawn from the kiln at an inclination to the vertical and a ribbon drawn from a deep kiln can be bent over a bending roller instead of being drawn through a vertical drawing tower.

The demands for high quality glass and higher rates of production have stimulated a continuous search by manufacturers for ways of creating better thermal and flow conditions in the drawing plant and numerous proposals to this end have been made in recent years.

Thus it has been proposed to heat bottom and side wall portions of the 'kiln from the outside to particularly high temperatures in order to reduce flow retardation along the walls. This expedience however does not produce conditions which are favorable to the production of high quality sheet glass. In fact it increases the risk of the drawn glass becoming contaminated by grains of refractory material or containing gas bubbles. The tendency for the refractory material to be corroded or eroded increases with increase of temperature of the refractory material.

SUMMARY OF THE INVENTION An object of the present invention is to provide for an improvement in the drawing of sheet glass by influencing the basic flow pattern within the body of molten glass.

Another object of the invention is to provide for an increased rate of drawing without the usual increased risk of refractory corrosion and erosion which this has hitherto entailed.

The objects according to the present invention are achieved in a process of manufacturing sheet glass by continuously feeding molten glass into a kiln to establish a continuous forward flow of glass to a drawing zone at which molten glass is continuously drawn upwardly from the surface of the molten glass in the kiln in the form of a ribbon, by the improvement in that in at least one place which in plan view is spaced inwardly from a boundary of the surface of the molten glass in the kiln, the molten glass in the kiln is heated to maintain at that place a thermal barrier formed by an upward flow of molten glass which rises to the molten glass surface from a location in the vicinity of a wall portion of or in the kiln so that molten glass behind the barrier is substantially prevented from flowing beneath such barrier.

The process according to the invention affords the important advantage that in at least one region around the meniscus at the root of the drawn glass ribbon, molten glass which is about to feed into the meniscus is given a lower viscosity and therefore a higher fiowability without having to flow against a heated kiln wall near the line defining the molten glass surface, where corrosion and erosion are most likely to occur.

The heated upwardly flowing molten glass rises to the surface of the molten glass against a quantity of molten glass which is excluded by the thermal barrier from access to the drawing zone. That excluded quantity of molten glass is continuously kept in movement due to a rather complex system of convection currents which exists on its side of the thermal barrier. Such currents help to avoid or reduce stagnation of any part of the molten glass such as is liable to produce devitrified grains of glass which might possibly become entrained towards the drawing zone.

It is a further important advantage of the process that molten glass which flows downwardly along a side or end wall of the kiln, behind the thermal barrier, and becomes cooler during such downward flow, is prevented from flowing beneath the thermal barrier and directly exerting a cooling action on the molten glass which, in plan aspect of the kiln, is in front of the barrier. Such a cooling action could adversely affect the velocity of flow of molten glass into the ribbon.

The invention makes an important contribution toward achieving a good stratification of the glass in the ribbon so that glass of high quality can be drawn at faster rates.

The stratification of a drawn glass sheet identifies its internal structure which is constituted by a series of discernible thin strata of glass due probably to the fact that the glass entering the ribbon cools at different rates throughout the thickness of the ribbon, the rate being highest at the surfaces and lowest at the central plane of the ribbon. These strata are generally, but far from exactly, parallel to the major faces of the ribbon. An ideal stratification is one in which the strata are perfectly flat and perfectly parallel to the major glass faces because this will result in glass having the best optical quality and particularly the smallest degree of optical distortion. Stratification is therefore improved when made to conform more clearly to the ideal.

The extent to which stratification is improved depends in part on the place or places at which a thermal barrier as referred to is created and on the horizontal extent of such barrier or barriers.

In certain embodiments of the invention, involving processes in which molten glass at the surface of the forward current feeds directly into the bottom of the ribbon at its front side and molten glass at a lower level of such forward flow rises at a location behind the drawing zone and forms an oppositely directed surface flow which feeds into the ribbon at its rear side, a thermal barrier is created at a position coinciding with the position where molten glass rises behind the drawing zone. A thermal barrier at that location has a particularly marked beneficial effect.

Over the important main part of the ribbon width, excluding its margins, the glass forming the rear side of the ribbon is wholly or mainly derived from the abovedescribed oppositely directed surface flow. If the glass of that oppositely directed surface flow is significantly less fluid, or more viscous, than the glass of the forward current surface flow, the drawing speed must be kept low if the drawn glass is to be of an acceptable quality. If the drawing speed is raised beyond a certain value, which depends to an appreciable degree on the flow resistance of the glass feeding the rear side of the ribbon, the ribbon will become deformed or will result in sheet glass which is optically very defective.

A drawing speed substantially higher than would normally be permissible can however be established if a thermal barrier in accordance with the invention is formed along a zone extending transversely of the kiln at a position such that some of the heated glass flowing upwardly at that zone serves to form the oppositely directed surface flow of glass into the rear side of the meniscus.

In such embodiments, the thermal barrier may be spaced from the rear end wall of the kiln, but this is not essential because, as hereafter exemplified, a part of that wall located beneath the glass surface can be shaped so as to extend inwardly towards or up to a position beneath the drawing zone so that the thermal barrier can be located over a submerged part of that rear end wall.

Depending on the extent of such thermal barrier across the width of the kiln, it may be possible for currents of glass to move from a location near the central part of the rear end wall of the kiln and to move around the ends of the thermal barrier towards the end portions of the drawing zone from which the margins of the ribbon are drawn. However any impurities thus drawn inwardly to the drawing zone are therefore directed to such end portions of the drawing zone and do not contaminate the central portion of the drawing zone from which the main usable portion of the ribbon is drawn.

Preferably such a thermal barrier which is located rearwardly of the drawing zone extends across the full width of the kiln, or at least over a portion of the kiln width which is at least co-extensive with the width of the ribbon. In that case the Whole or substantially the whole of the body of molten glass which is adjacent to the rear end Wall of the kiln near the molten glass surface and which would normally be pulled inwardly to the drawing zone by the currents caused by the drawing operation, is effectively isolated by the thermal barrier.

A significant improvement in the drawing process can however be achieved by forming a thermal barrier in accordance with the invention at a location which in plan aspect of the kiln is situated adjacent a boundary of the surface of the molten glass in the kiln and at which there is surface fiow of molten glass towards an edge or margin of the ribbon. By means of a thermal barrier so placed, the flowability of the molten glass feeding the corresponding edge or margin of the ribbon is improved due to the heating of such glass and due to a reduction in frictional restraint. The flow of glass feeding the edge or margin of the ribbon is shielded from currents of more viscous glass which has been cooled by contact with the side wall of the kiln near the glass surface. In consequence, the Width of the marginal portions of the ribbon which have to be discarded when the ribbon is cut is reduced.

In the case that a thermal barrier is located adjacent a side boundary of the glass surface, as above referred to, it is of course preferable for a similar thermal barrier also to be maintained adjacent the other side boundary of such surface so that similar thermal and flow conditions determine the formation of both side edge or marginal portions of the ribbon.

It is of course very advantageous for thermal barriers according to the invention to be maintained adjacent the two side boundaries of the molten glass surface and also rearwardly of the drawing zone. In that case the glass feeding the rear side of the ribbon and its margins can be kept in a highly flowable condition commensurate with that of the main forward surface flow which directly feeds the front side of the ribbon, and the permissible drawing speeds are maximized since they are determined by the fluidity of the forward surface current.

According to certain important embodiments of the invention there is at least one thermal barrier functioning as above referred to and maintained above a threshold completely immersed in the molten glass. The threshold serves positively to localize the ascending currents resulting from the local heating of the glass. The body of glass which is behind the threshold tends to be kept in stable circulatory movement around a horizontal axis, which also helps to avoid or reduce any tendency for an accumulation of devitrified grains to be formed in the body of glass. Moreover the threshold also serves as a mechanical barrier against inward displacement beneath the thermal barrier of any such grains of devitrified material or of any refractory grains with which the body of glass might possibly become contamined.

Inasmuch as the threshold is submerged and is therefore not in contact with the air above the molten glass, the threshold is not as likely to suffer corrosion by the molten glass currents as is the refractory material at the level of the glass surface.

It is advantageous for the upward flow of glass forming a thermal barrier to commence below the level or the top of the threshold, if such is provided, so that the glass rises against the threshold and continues its upward movement above the level of the top of such threshold. The threshold then better serves to stabilize the upward flow of glass.

The side faces of the threshold may be vertical, or inclined to the vertical, or one such face may be vertical and the other inclined. The height and shaping of the threshold influences the direction of the upward flow paths of the glass forming the thermal barrier and thus the flow pattern of the glass currents feeding the ribbon.

Where a threshold is provided, the threshold may be hollow and the heat required for creating the thermal barrier can be generated by heating members disposed in the space within the threshold. The heating means is thus shielded by the threshold from direct contact with the molten glass such as would restrict the choice of heating means. As heating means, use can be made, for example, of gas or oil burners or electrical resistances.

Use can alternatively be made of a threshold constituted by a single solid wall. In that case the heat for creating the thermal barrier may be generated at the bottom of such wall.

There are also advantages, when making use of a submerged threshold, in using heating means incorporated in, or forming part of, the, or a, wall forming the threshold or part of the threshold. For example, such wall may be formed in part of one or more electrically conductive refractory elements through which electric current can be passed to generate heat by the Joule elfect. This method aids the creation of a strong thermal action which is localized where it is most elfective for preventing flow of glass currents across the top of the threshold.

For maintaining the thermal barrier at the location of a threshold, it is also possible to use heating means which is actually in contact with the molten glass adjacent the threshold. The use of heating means in contact with the molten glass but not actually forming part of the threshold is useful for generating heat at a well defined zone or zones while leaving the threshold free from any direct heating function and thus widening the choice of its design specifications. By way of example, use can be made of heating elements disposed on a side face and/or on the top face of the threshold.

Alternatively, or in addition, use can be made of heating elements, e.g. electrical resistance heaters, which are located within the body of molten glass and spaced from the threshold. Such an arrangement is of advantage for reducing any risk of corrosion of the threshold. Where it is required to raise to a certain level the temperature of the glass in a particular region spaced from the threshold, this arrangement enables that heating to be effected with a lower energy consumption than if the heat were to be generated in or immediately on the threshold.

In a system in which the kiln is provided with a threshold, the latter can be upwardly extended by a plate to increase the height of the mechanical barrier preventing inward flow of impurities such as devitrified material or bubbles. Such plate can be made of metal, e.g. molybdenum. It is advantageous for the top of the plate to be disposed as close as possible to the free surface of the molten glass in the kiln.

A thermal barrier at one or more locations as required by the invention can of course be created by generating heat within the kiln, by electrical resistance heaters or other means, regardless of whether or not a threshold is provided at that place or places. However, the provision of a threshold is of particular interest because it helps to stabilize the thermal barrier.

A very advantageous way of creating a thermal barrier is to pass an electric current or currents through the molten glass at the region to be heated, between suitably placed electrodes. This type of heating system produces the required heat directly in the molten glass itself and the glass can be kept at a required high temperature while the electrodes are at a lower temperature level which may be low enough substantially to avoid any risk of corrosion of the electrodes by the molten glass.

Electrodes for use in a heating system using the electrical conductivity of the molten glass can be in the form of plates or rods. It is very advantageous however, to use electrodes formed by pools of molten metal or molten metal salt. Molten metal or molten salt electrodes can be of large surface area, producing the additional benefit of a very low frictional restraint on the flow of molten glass in contact with the electrodes.

One very satisfactory heating system makes use of electrodes one at least of which is disposed above a threshold completely immersed in the molten glass.

Another possible arrangement of electrodes which is very suitable in certain cases is an arrangement of the electrodes on opposite sides of such a threshold. By using electrodes disposed in that manner, a considerable volume of molten glass covering the threshold can be directly heated for a comparatively low energy consumption. It is convenient in such a system to use electrodes with large surface areas in contact with the glass so that a given heating elfect can be realized with a low current density, which is desirable for avoiding bubble formation in the glass.

In the case that a metal plate is used to increase the height of the physical barrier formed by the threshold and heating is achieved by an electric current or currents passing through the glass between electrodes disposed on opposite sides of the threshold, the plate will define an equipotential surface in the electric field and its shape may be selected to achieve a required preferential direction of the electric current.

In certain embodiments of the invention the molten glass in the kiln is locally heated to create a thermal barrier by passing electric current through the molten glass between electrodes one of which is disposed beneath the location at which the ribbon is drawn from the surface of the molten glass in the kiln. In this way a thermal barrier can be maintained very close to the drawing zone. In one very satisfactory arrangement, a draw bar is provided beneath the drawing location and an electrode in the form of a quantity of molten metal or molten metal salt is held in this draw bar. The presence of a pool of molten metal or molten metal salt at that location is beneficial from the point of view of the low frictional restraint which it imposes on the molten glass flowing into the meniscus.

Another process feature which is of value involves the production of a surface current of molten glass over the thermal barrier and in a direction away from the drawing zone, and the withdrawal of surplus glass from a region behind such barrier. The outward surface current supplements the action of the thermal barrier in countering any tendency for impurities to be entrained inwardly towards the drawing zone. Such an outward surface current can be produced by withdrawing glass via at least one skim hole in a boundary wall of the kiln at a region behind the or a thermal barrier.

The invention includes apparatus for use in drawing sheet glass, the apparatus including a kiln having a feed end at which the kiln can be continuously fed with molten glass, and means for continuously drawing a ribbon of glass upwardly from the surface of the glass at a drawing zone in the kiln, in which there is means for locally heating the molten glass in the kiln in at least one place which in plan aspect of the kiln is spaced inwardly from a boundary of the molten glass surface in order to maintain at that place a thermal barrier formed by an upward flow of molten glass which rises to the surface from a position in the vicinity of a wall portion of or in the kiln whereby the wall portion serves to prevent molten glass behind said barrier from flowing beneath it.

The provision of local heating means according to the invention permits the creation of a hot zone which renders the glass feeding the ribbon from at least one region around the meniscus more fluid, while forming a thermal barrier preventing cooler glass from flowing into the ribbon from the wall portion of the kiln behind the thermal barrier. Consequently glass having good stratification and which is generally of high quality can be drawn at a faster rate by apparatus according to the invention.

In preferred apparatus according to the invention, the drawing zone is spaced from that boundary of the surface of the molten glass in the kiln which is opposite the feed end of the kiln and local heating means which is provided for maintaining an upward flow of glass at a location which in plan aspect of the kiln is between the drawing zone and that opposite boundary reached by the surface of the molten glass in the kiln when the apparatus is in use. The advantage of this and other optional apparatus features hereinafter referred to will be appreciated from the statements which have been made hereinbefore as to the advantages of the corresponding process features.

In the most important embodiments of the invention, means are provided for maintaining the upward flow of glass between the drawing zone and the opposite glass surface boundary, such upward flow occurring across the full width of the kiln or at least over a portion of the kiln width which is substantially co-extensive with the width of the ribbon, this width being determined by the position of the conventional edge rollers between which the margins of the drawn ribbon are drawn.

Advantageously however, apparatus according to the invention can include means for maintaining the upward flow of molten glass in at least one place which in plan aspect of the kiln is adjacent a side boundary to which the surface of the molten glass in the kiln extends when the apparatus is in use. It is particularly beneficial for the apparatus to incorporate means for maintaining such an upward flow of glass adjacent each side boundary of the molten glass surface and in certain apparatus according to the invention, in addition to the heating means for creating the upward flow at those places, means as above referred to are provided for creating a hot zone directly rearwardly of the drawing zone. In that case the flow of molten glass into the rear side of the ribbon and into its edges or margins can be promoted to enable very high drawing rates to be achieved.

Apparatus according to the invention may alternatively including a shallow pot, the drawing means being adapted to draw glass from the full depth of the molten glass in the kiln. In this case, the means for creating a local hot zone in accordance with the invention can be arranged so that it is effective for maintaining an upward flow of molten glass at that zone throughout the full depth of the molten glass in the kiln.

Apparatus according ot the invention may alternatively be of a type including a deep tank, the drawing means being adapted to draw glass from an upper portion of the molten glass in the kiln. In this case the means for creating a local hot zone in accordance with the invention is arranged so that it is efiective for maintaining an upward flow of molten glass at that zone, at least in the upper portion of the depth of molten glass in the kiln, and the apparatus is arranged so that the upward flow of molten glass at the hot zone takes place from a location in the vicinity of at least one wall portion so that flow of molten glass beneath the thermal barrier is substantially prevented.

Certain forms of apparatus according to the invention incorporate a threshold located so as to be completely immersed in the molten glass in the kiln when the apparatus is in use, and so that heating means maintains an upward flow of molten glass above such threshold. Preferably heating means are provided for maintaining an upward flow of molten glass from a location below the level of the top of the threshold so that the upward flow occurs against the threshold and continues above its top most level.

Advantageously the portion of the bottom of the kiln disposed behind a threshold is higher than the portion in front of the threshold. In that case the depth of unused glass behind the threshold is reduced. In the case of a deep kiln process there is the further advantage that the molten glass at the lower levels of the kiln in front of the threshold can be more effectively cooled, which promotes a more positive downward movement of the glass forming the submerged return current.

Advantageously, the apparatus includes a threshold which is hollow and means are provided for generating within the threshold the heat required for creating the local hot zone at that spot.

In other advantageous embodiments, the threshold is constituted by a single solid wall and the means for creating the local hot zone at that spot is arranged to generate heat at the bottom of such wall.

For creating a local hot zone, certain apparatus according to the invention is provided with heating means which is incorporated in or forms part of a wall forming at least part of the threshold. By way of example, such wall may be formed in part by one or more electrically con ductive refractory elements, such as one or more tin oxide bricks, through which electrical heating current can be passed. Such heating means can have a surface which is flush with or which is set back or projects from an adjacent part of the threshold surface.

Preferably, the apparatus incorporates heating means for maintaining a local hot zone and a threshold at that zone, such heating means being located within the kiln so as actually to be in contact with the molten glass, adjacent or spaced from such threshold. By way of example, heating elements may be located above the threshold so that the heating of the glass in that region can be more intense or the temperature gradient at that region can be different from that which would be possible if the heating elements were located below or in the threshold. Of course, heating elements can also be provided at a lower level for giving the glass adjacent the threshold an upward impulse.

According to a very satisfactory arrangement, local heating means are provided for heating molten glass in the kiln to maintain the upward flow of molten glass, which heating means generates heat directly within the kiln.

In the most preferred embodiments of apparatus according to the invention, the heating means for maintaining the thermal barrier is composed of electrodes between which electric current can be passed through molten glass in the kiln. Such electrodes may be in the form of plates or rods but preferably they are formed by pools of molten metal or molten metal salt. In one arrangement, there is at least one electrode disposed above a threshold, but preferably the heating means includes electrodes located on opposite sides of the threshold.

It is advantageous to place an electrode beneath the location at which the glass ribbon is drawn from the surface of the molten glass in the kiln when the apparatus is in use. By way of example, a draw bar may be provided at that location and may incorporate or hold an electrode. An electrode located beneath the drawing location as aforesaid can in any case be incorporated in or held by an element which is integral with or connected to the rear end wall of the kiln. In some cases the absence of a free path behind such electrode, along which molten glass can flow upwardly into the path of the electric heating current from a lower level in the kiln, is beneficial in promoting better thermal and dynamic flow conditions within the kiln.

The use of electrodes for heating the molten glass avoids the necessity for transmitting heat energy through refractories with the consequent risk of their being heated to such an extent as to render likely the corrosion of the refractories by the molten glass. Moreover there is less risk of inducing turbulent, uncontrollable convection currents in the glass than when using heating means which relies entirely on convection currents in the glass for heating the glass in the hot zone.

The use of electrodes disposed on opposite sides of the threshold enables a large volume of glass to be heated at the hot zone, giving rise to a very marked upward current. Electrodes with large surface areas can be conveniently located in those areas, which is desirable for avoiding high current densities and the formation of bubbles. It is advantageous for the top of the threshold to be fairly close to the surface level of the molten glass so as to give a relatively high current density over the threshold and to make the threshold as effective as possible as a barrier against the inward movement of impurities into the drawing zone.

When using electrodes on opposite sides of the threshold and at the bottom of the kiln, it is advantageous for the bottom of the kiln to be at a higher level behind the threshold than in front of the threshold because in that case, in addition to the advantages attributable to such a difference in level as hereinbefore referred to, there is the advantage that the electrodes can be closer together.

In the case where electrodes are used, at least one electrode may be made of a solid metal or of an electrically conductive refractory material, preference being given to the refractory precious metals, molybdenum, tungsten and SnO doping agents being incorporated if required. It has been found that these materials behave satisfactorily in molten glass at elevated temperature even when an electric current passes through the surface of the material in contact with the glass. Moreover, the solid electrodes can be of a shape selected so as to achieve a predetermined current density distribution.

Special advantages attach to the use of electrodes composed of molten metal or molten metal salt. If the molten metal or salt is heavier than the molten glass, the electrodes are disposed beneath the molten glass and assist in reducing the frictional flow resistance experienced by the molten glass in the kiln. It is possible to use electrodes composed of molten metal or molten metal salt of lower specific gravity than the molten glass. Such electrodes do not in any way impede the molten glass surface currents. Such electrodes can be readily replenished in course of time and their thicknesses can be altered when required. Moreover, their electrical properties can be modified, by changing their chemical compositions, without interrupting the sheet glass production.

Suitable molten metals which are heavier than glass are tin and lead. These metals have a high electrical conductivity.

According to one optional but very advantageous feature of the invention a reservoir is provided for holding an electrode composed of molten metal or metal salt in contact with the molten glass in the kiln, and such reservoir has an extension leading to a cooler region where metal or metal salt occupying such extension can be connected to a conductor cable. This solves most of the problems involved in maintaining a good electrical connection between a cable and an electrode in a very high temperature region, e.g. at the drawing zone side of a threshold. By way of example, a molten tin electrode may be held in a reservoir with an extention channel leading to a cooler zone so that the tin in this channel is in a solid or at least a cooler state, and the cable may be connected to the solid or cooler tin. A molten metal salt electrode may likewise make contact with a body of the same or another metal salt in solid or at any rate cooler, condition, provided that the cooler salt is sufliciently electrically conductive.

The kiln of an apparatus according to the invention may be provided with at least one skim opening located in a boundary wall thereof opposite the location of a means for creating a local hot zone. When the apparatus is in use, a small amount of glass can be withdrawn contin- 10 uously or intermittently from the surface of the molten glass via such skim opening. The withdrawal of glass from the surface in that way induces an outward current through the top of the thermal barrier to serve as an additional check against the inward flow of impurities.

The present invention further relates to sheet glass produced as described above and itself possessing novel characteristics.

Sheet glass is composed of seams or strata of glass having different refractive indices. The optical quality of the sheet glass depends to a large extent on the relative distribution of the differently refractive seams within the sheet. If there is an appreciable interpenetration of seams of different refractive indices, the sheet tends to give a distorted appearance to objects viewed through the glass under various conditions, even if the main faces of the sheet are optically flat and parallel. The seam pattern depends at least in part on the spatial distribution of the currents of glass of different viscosities feeding the ribbon during the drawing process. The seam pattern existing in any given sample of drawn glass can be seen in an enlarged photographic image of a cross section of the sample cut normal to the line or draw.

Thus, a further object of the present invention is a sheet glass composed of seams of glass of different refractive indices distributed in a way such that light rays entering the sheet glass through either of its faces, over a wide range of angles of incidence, are subject to very little if any refraction within the sheet.

Sheet glass according to the invention is characterized in that the distribution of seams of glass of different refractive indices in a cross section which extends over the full width of the drawn ribbon and which is normal to the line of draw is discernible in any such cross section as a pattern of mainly substantially parallel contour lines forming or visually suggesting a pattern of fiat ellipses one within another, and in that this distribution is such that there is no abrupt substantial change of refractive index from one seam of glass to another such as to cause a marked break in the continuity of the parallel interference fringes when the sheet is examined by means of an interferential microrefractometer using a light beam which is projected through the sheet glass parallel with its main faces.

Sheet glass according to the invention has been found to have excellent optical characteristics. The sheet glass gives rise to very little if any apparent distortion of objects viewed through the glass even at shallow or changing angles relative to the plane of the sheet.

When sheet glass according to the invention is examined in cross section normal to the line of draw to detect the pattern of contour lines formed by the juxtaposition of seams of glass of different refractive indices, the contour pattern is seen to be substantially free of crossing lines. The contour lines are mainly substantially parallel and may substantially all describe figures approximating flat ellipses, but this is not essential. However, in all cases the system of contour lines as a whole at least suggests to the eye a basic pattern of flat ellipses due to the fact that there are numerous contours which form flat ellipses or which at least form major parts of flat ellipses.

Techniques for examining and photographically recording the pattern of distribution of differently refractive seams of glass within a drawn glass sheet are well known in glass technology and have been used extensively in relation to samples of sheet glass hitherto available and drawn by various processes. In general the known sheet glass exhibits under such examination a system of contour lines which cross at one or more locations and/or which do not fall into any kind of elliptical pattern.

The pattern formed by the contour lines in sheet glass according to the invention is not however the only important characteristic of such glass. A further important characteristic is the absence of or substantial change in refractive index from one seam of glass to another across the thickness of the sheet. The refractive index variations through the thickness of the sheet are so small or gradual that they do not cause a marked break in the continuity of the interference fringes when the sheet is examined by means of an interferential microrefractometer.

An interferential microrefractometer is an instrument in which a beam of light is projected through a sample to be examined, and is divided into unequally retarded parts to give rise to a pattern of interference fringes. When a sample of sheet glass according to the invention is examined by means of such an instrument, the sheet being located so that the light beam enters one edge face of the sample and emerges from the opposite edge face, such opposed edge faces being parallel, no marked faults appear in the interference fringes whatever be the orientation of the sheet about the axis of the light beam.

A very suitable type of interferential microrefractometer is the one developed by Nomars-ki. The Nomarski interferential method is described, for example, in the publication Techniques de lIngenieur of the year 1961, Chapter R3422, paragraphs 6.3 to 6.6, particularly paragraph 6.52, under the heading Objectif interferential a prisme de Wollaston. This paper is published by Techniques de IIngenieur, 21, rue Cassette, Paris VI, France.

The Nomarski instrument incorporates a slit light source and a combination of prisms and polarizing filters to create an interference pattern in the form of a series of parallel lines or bands. When using such an apparatus for testing a sheet of glass the sheet should be located in a plane which is non-parallel to the interference lines or bands. The sheet can for example be located in a plane which is at 45 to such lines or bands. A sheet of glass according to the invention does not however give rise to a marked break in the interference lines or bands Whatever be the orientation of the sheet about the axis of the light beam so that the sheet can be rotated about such axis during the test and no marked faulting of the interference lines or bands will appear during rotation of the sheet through 360.

Sheet glass according to the invention as above defined can be produced by the drawing process according to the invention. By means of such a process, sheet glass according to the invention can be produced with consistent reliability, particularly when a thermal barrier is created directly rearwardly of the drawing zone. It is supposed that one result of establishing and maintaining a thermal barrier or two or more thermal barriers as hereinbefore described is that the flow profile existing in the mass of molten glass in the kiln at a position in front of the drawing zone is substantially maintained over the part of the kiln in which the glass currents change direction to feed into the front and rear sides of the ribbon.

Preferred embodiments of sheet glass according to the invention are characterized by the substantial absence of brush lines.

One feature of much of the known sheet glass is the presence on at least one face of the sheet glass of defects known as brush lines." Such defects can be observed and recorded by interferometry, using the known Fizeau fringes, or by examining a reflected image of the glass face, obtained by causing a light beam to be reflected from the face onto a light-diffusing screen as will hereafter be more particularly described.

Sheet glass according to the invention which is further characterized by the substantial absence of brush lines can be produced consistently and reliably by a process according to the invention as hereinbefore defined if a thermal barrier is maintained at a location which is directly rearward of the drawing zone. The effect of such a thermal barrier is to protect the current of glass which rises and feeds into the rear side of the glass ribbon from contact with the rear end wall of the kiln and it is presumably this fact which entirely or mainly accounts for the substantial absence of brush lines from the drawn sheet glass.

12 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational transverse cross section of part of a Pittsburgh-type machine, taken on line II of FIG. 2, provided with an embodiment of apparatus according to the invention.

FIG. 2 is a plan view of the machine portion shown in FIG. 1.

FIG. 3 is an elevational longitudinal cross section of part of a Colburn-type machine provided with an embodiment of apparatus according to the invention.

FIG. 4 is an elevational longitudinal cross section of part of another Colburn-type machine, taken on line IV-IV of FIG. 5, having another embodiment of the invention.

FIG. 5 is a broken plan view of the machine part shown in FIG. 4.

FIG. 6 is an elevational longitudinal cross section of part of another Pittsburgh-type machine having an embodiment of the invention.

FIG. 7 is an elevational longitudinal cross section of part of another Pittsburgh-type machine equipped with an embodiment of the invention.

FIG. 8 is an elevational transverse cross-sectional view of part of the machine represented in FIG. 7, taken on the line VIII-VIII of FIG. 7.

FIG. 9 is an elevational longitudinal cross section of part of another Colburn-type machine.

FIG. 10 is an elevational longitudinal cross section of part of another Colburn-type machine.

FIG. 11 is a vertical longitudinal cross section of part of another Colburn-type machine.

FIGS. 12 to 17 are elevational detail views of portions of the kilns of six different glass-drawing machines in cross section parallel to the longitudinal axes of the kilns.

FIGS. 18 and 19 are elevational longitudinal cross sections of parts of Pittsburgh-type machines provided with further embodiments of the invention.

FIG. 20 is a reproduction of an anamorphosic photograph or striascope, of a sample of sheet glass according to the invention.

FIG. 21 is a photographic reproduction of the interference fringes formed by light rays projected through the sample of sheet glass represented by FIG. 20 during testing of such sample in an interferential microrefractometer according to the known method of Nomarski.

FIG. 22 is a reproduction of a striascope, as above referred to, of a sample of sheet glass drawn by a classic prior art Pittsburgh-type drawing process.

FIG. 23 is a photographic reproduction of the interference fringes formed by light rays projected through a sample of sheet glass represented by FIG. 22 and drawn by a classic Pittsburgh-type drawing process, during testing of such sample in an interferential microrefractometer according to the Nornarski method hereinbefore referred to.

FIG. 24 is a photographic reproduction of a typical brush line pattern detected by photographic methods on a face of a sample of sheet glass drawn by a classic Libbey-Owens type process. 1

FIG. 25 is a schematic view of a striascopic apparatus for photographically recording the homogeneity of the glass within the cross section of a glass sheet.

FIG. 26 is a schematic view of the optical system of an intcrferential microrefractometer for performing the Nomarski method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The machine shown in FIGS. 1 and 2 includes a kiln constituted by a deep tank 1 for holding a quantity of molten glass. The lower portion of the tank is defined by a sole 2, a lower rear end wall portion 3 and lower side wall portions 4 and 5. At its upper portion, the horizontal dimensions of the tank are extended by horizontal wall portions 6, 7 and 8 which extend outwardly from the walls 3, 4 and 5, respectively, and support an upper end wall 9 and upper side walls 10 and 11.

When the apparatus is in use, molten glass is continuously fed into the tank via its end (not shown) remote from the end wall 3, 9 so as to maintain the surface 12 of the molten glass at the indicated level within the tank while glass is drawn upwardly from such surface in the form of a continuous ribbon 13 having lateral, or side, edges 14 and 15. The line of intersection of the glass surface 12 with the tank walls is referred to as the flux line.

Within the tank there are three horizontally extending electrodes 16 disposed one over the other parallel to, but inwardly spaced from, the upper end wall portion 9 of the tank. Three similar groups each of three horizontally extending electrodes 17 are disposed parallel to, but inwardly spaced from, the upper side wall portion 10 and three similar groups each of three horizontally extending electrodes 18 are disposed parallel to, but inwardly spaced from, the upper side wall portion 11 of the tank. The electrodes 17 are connected to a source 19 of direct electric current, the central electrode 17 being connected to one pole of such source and the top and bottom electrodes 17 being connected to the other pole thereof. The other group of electrodes, i.e. the electrodes 16 and 18, are similarly connected to electric current sources which, however, are not shown to avoid complicating the drawmgs.

The drawing machine incorporates, over the tank, a drawing chamber and tower section through which the ribbon of glass is drawn by rollers. These and other parts of the machine are in accordance with well known prac tice and need no explanation or illustration.

The electrical potential applied to the electrodes of each of the groups 16, 17 and 18 is such that a continuous electric current passes through the molten glass in the kiln between the electrodes at different potentials in that group. The voltage is such that at no place is the current density in the molten glass in excess of 0.4 amps/ emf, and by virtue of the passage of the electric current through the molten glass the temperature of the glass in the vicinity of each group of electrodes is about 40 C. higher than it would otherwise be. In consequence a thermal barrier is established and maintained at the site of each group of electrodes.

At this site there is a continuous upward flow of molten glass which rises from a location in the vicinity of the underlying horizontal wall portion 6, 7 or 8, as the case may be. Some of the upwardly flowing molten glass flows inwardly along the surface of the molten glass in the tank toward the meniscus at the bottom of the ribbon 13, and the remainder of such upwardly flowing molten glass flows outwardly toward the adjacent wall portion 9, 10 or 11, as the case may be.

Behind each thermal barrier, i.e. between the barrier and the adjacent upper wall, there is a relatively cool zone of molten glass and the shaping of the side and end walls of the kiln to provide the horizontal wall portions 6, 7 and 8 underlying the thermal barriers prevents glass from such relatively cool zones from flowing inwardly beneath the barriers. In consequence there is appreciably less risk that devitrified glass grains which may form adjacent the wall portions 6, 7, 8, 9, 10 and 11, and that grains of corroded refractory material which may separate from the flux line blocks of the wall portions 9, 10 and 11, will pass inwardly into the currents of glass feeding the meniscus at the bottom of the glass ribbon 13.

The quantities of glass in the relatively cool zones behind the electrode groups are circulated about horizontal axes and this in turn reduces the risk of devitrification and bubble formation occurring in the molten glass at lower levels of such zones. Moreover, the local heating of the glass at the sites of the electrode groups reduces the viscosity of the glass flowing into the rear side and the side edge margins of the ribbon so that the width of the marginal portions of the ribbon which exceed the maximum permissible thickness is reduced for any given speed of drawing and glass of a given quality standard can be drawn at a faster rate than in conventional processes. The increase in the usable width of the drawn ribbon when using a process according to the invention can be for example as much as 10 cm. when the above-mentioned 40 C. temperature increase is achieved.

In a modification of the process described with reference to FIGS. 1 and 2, only the electrodes 16 were used. In that case the maximum drawing speed was somewhat less due to the restraint imposed by the somewhat higher viscosity of the molten glass feeding the marginal edge portions of the glass ribbon and the width of the marginal edge por tions of the ribbon which had to be discarded as waste was greater due to contamination of the glass in those regions by devitrified grains. The quality of the sheet glass at the operative drawing speed was however considerably better than during an identical operation of the plant, but Without using the electrodes 16, and of course without using electrodes 17 and 18.

According to another modification, only the electrode groups 17 and 18 were used. In that case the maximum drawing speed at which a ribbon having a given standard of flatness and uniformity of thickness could be drawn was higher than in a conventional process without a thermal barrier, the width of the marginal portions which exceeded the maximum permissible thickness being normal. However it was found that there were appreciably more defects in the central part of the drawn ribbon, due to the presence of devitrified grains and bubbles, than when the electrodes 16 were used.

Reference is now made to FIG. 3 illustrating part of a standard Colburn-type glass-drawing machine equipped for performing a process according to the invention. The machine includes a shallow drawing kiln, or pot, 20 defined by a sole 21, a rear end wall 22 and side walls only one of which, wall 23, is visible in the drawing. The pot is supported by piers 24 and 25. Molten glass is fed continuously into and along the pot from a glass melting furnace towards the rear end wall of the pot and a ribbon of glass 26 is drawn continuously from the surface 27 of the glass in the pot and passes around a bending roller 28 into an annealing lehr. The annealing lehr, the rollers employed for conveying the glass ribbon through the lehr, the lip tiles and other standard parts of the machine are not illustrated and need no explanation as they are in accordance with standard practice.

The pot 20 is shallow so that the glass entering the continuously drawn ribbon 26 is drawn from the full depth of the molten glass in the pot.

Within the pot 20, at a location between the drawing zone and the rear end wall 22, there are two electrical resistance heaters 29 and 30 extending transversely across the full width of the pot, parallel to both the drawing zone, and the rear end wall.

During the glass drawing process, electric current is passed continuously through the resistance heaters 29 and 30 so as to effect local heating of the molten glass in the corresponding region of the pot. The heaters icnrease the local temperature of the glass by about 50 C. In consequence there is a continuous upward flow of molten glass to the surface 2'7 in the vicinity of the heaters. Such upward flow of molten glass takes place from the vicinity of the directly underlying portion of the sole 21 of the pot.

The basic flow pattern of the molten glass in the pot, in the vertical longitudinal plane of FIG. 3, is represented by arrows in the drawing. It will be noted that the flow pattern differs from that which takes place in conventional shallow pot processes in that the flow of glass into the rear face of the ribbon does not take place from the rear end wall of the pot but from a location which is spaced inwardly 15 from that wall along the surface of the molten glass, that location being determined by the position of the heaters 29 and 30.

The molten glass situated behind the vertical transverse plane containing the heaters is substantially excluded from the flow of glass to the meniscus by the thermal barrier constituted by the ascending current of glass around the heater. Flow of molten glass beneath the thermal barrier cannot take place to any notable extent because the upward flow of molten glass commences from the bottom of the kiln.

The molten glass behind the thermal barrier is kept in movement by continuous circulatory convection currents. Any grains of devitrified glass which may form in the vicinity of the rear end wall or and/or any grains of corroded refractory material which may separate from the refractory wall in that region are virtually excluded from access to the drawing zone by such thermal barrier. By virtue of this fact and the low viscosity of the glass feeding the rear face of the ribbon, the maximum drawing speed at which good quality sheet glass can be drawn is increased by about 30% without any attendant reduction in glass quality.

Reference is now made to FIGS. 4 and .5 where again only such parts of the machine are illustrated as are required for describing and understanding the invention. FIGS. 4 and show part of a pot 31 into which the molten glass is continuously fed from a glass melting furnace, this pot being supported on piers 33 and 34. The region 35 above the pot is in practice enclosed, this being the region within the conventional drawing chamber, but the drawing chamber with its lip tiles, the annealing lehr, the conveying rollers by which the glass ribbon is supported and conveyed through the lehr, and other parts which are standard in this type of machine, have not been shown as they are not relevant to an understanding of the invention. The only part located above the pot which is illustrated in the drawing is the bending roller 36 around which the glass ribbon passes before entering the annealing lehr.

The pot 31 has a rear end Wall 37 and a sole composed of sections 38 and 39 adjacent, and at respective sides of, a threshold 40 which extends transversely across the pot and is formed by walls 41, 42 and 43. The threshold divides the lower portion of the pot into an end compartment 44 and a front compartment 45.

A series of electrical resistance heaters 46 extend upwardly through the threshold so as to penetrate into the molten glass in the pot and are connected to a source of electric current (not shown). The lower portions 47 of these electrodes, located outside the pot, are sheathed with refractory material.

Glass is drawn upwardly from the surface of the molten glass in the pot so that a meniscus 48, the rear and front faces of which are designated 49 and 50, is established at the molten glass surface, leading into the glass ribbon 51 having rear and front faces 52 and 53, respectively. It is to be understood that as used herein, rear is the downstream side with respect to the direction of flow of molten glass from the melting tank and front is the upstream side.

In an actual embodiment of a process using apparatus as illustrated by FIGS. 4 and 5, the electrodes 46, which were located at intervals of 25 cm. across the pot, were connected to a voltage source such that the total power output acting to heat the molten glass above the threshold 40 was 30 kw. This power output maintained the glass in the region above the threshold at a temperature 30 C above what it would otherwise be.

It was observed that this local heating of the molten glass produced a stable upward flow of molten glass along the walls 41 and 43 of the threshold and continuing up to the surface of the molten glass in the pot. The currents of molten glass ascending along the wall 41 flowed outwardly toward the rear end wall 37 of the pot and descended along this wall so that the quantity of glass occupying the compartment 44 was maintained in circulatory movement and was effectively isolated from the surface flow of glass feeding the rear side 49 of the meniscus 48. This surface flow of glass into the rear side of the meniscus derived wholly, or at least mainly, from the upward flow of molten glass along the front wall 43 of the threshold.

Grains of corroded refractory material or of devitrified glass forming in the compartment 44 were. entirely, or almost entirely, excluded from access to the drawing zone, i.e. from the region at the surface of the molten glass in the pot where the meniscus 48 was formed.

This latter result was moreover further promoted by the fact that the rate of feed of molten glass to the pot from the glass melting furnace was slightly (about 1%) in excess of the rate of withdrawal of molten glass from the pot into the ribbon 51. The excess glass was continuously discharged through a series of skim holes 54 formed in the end wall 37 at the level of the molten glass surface and leading into flow channels 55. A similar result could be achieved by forming the holes 54 in the end wall 37 at a lower level. Whatever be the level of the skim holes, the discharge of glass through these holes could be intermittent instead of continuous.

FIG. 6 shows part of a Pittsburgh-type machine which includes a tank 60 having a sole 61, a rear end wall 62 and side walls only one of which, wall 63, is visible in the drawing. Preferably the end wall 62 is formed from a plurality of refractory elements of different compositions in order that the temperature of the mass of molten glass in the tank can be better controlled, but that is not essential. The tank is kept filled with molten glass up to a level 64, just sufiicient to maintain a slow continuous discharge of molten glass via an overflow 65 at the top of the end wall 62.

The end wall 62 is specially formed to provide a threshold 66 completely submerged in the molten glass in the tank and extending across the full width of the tank. The top faces of the portions of the wall 62 which are adjacent the threshold 66, and which are likewise submerged in the molten glass, support respective electrodes 67 and 68 preferably formed of tungsten.

The tank 60 is surrounded at the top by the usual drawing chamber which is bounded at the rear and front by rear and front L-blocks 69 and 70, respectively, and through which the molten glass is drawn continuously in the form of a ribbon 71. The drawing chamber is surmounted by a tower section through which the ribbon is drawn upwardly by entraining rollers. These and other parts of the machine, such as the coolers provided within the drawing chamber and the edge rolls between which the edges of the ribbon are gripped a short distance above the surface of the molten glass in the tank are not shown as they are in accordance with well established practice and are not relevant for the purpose of describing the invention.

The electrodes 67 and 68, which extend across the full width of the tank 60, are connected to the poles of an alternating electric current source, diagrammatically represented in the drawing, so that an electric current passes continuously through the molten glass between such electrodes. The electric potential is such that the current density within the glass is nowhere in excess of 0.6 amps/ cm.

The glass ribbon is fed by a forward flow of molten glass constituting the upper part of the molten glass mass in the tank. Molten glass at the surface region of this forward flow feeds directly into the front face of ribbon as indicated by arrow 72, whereas some of the glass at a lower level in such forward flow continues past the drawing zone and then flows upwardly to the surface of the glass in the vicinity of the threshold 66, as indicated by arrow 73, before flowing back to the meniscus along the surface region of the molten glass mass and then into the rear face of the ribbon. 

